1. Field of the Invention
The present invention relates to test and measurement equipment. More specifically, the present invention relates to equipment used to test infrared equipment.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Infrared detection devices such as forward looking infrared (FLIR) devices detect heat energy in the form of infrared radiation emanating from an object. Optimal performance of these devices requires testing and calibration prior to use in the field. To minimize the test equipment required to project a temperature pattern for an infrared detection device, a target was developed by M. P. Wirick and J. P. Wright of Hughes Aircraft Company. As described in U.S. Pat. No. 4,387,301 issued Jun. 7, 1983, and entitled TARGET FOR CALIBRATING AND TESTING INFRARED DETECTION DEVICES, the teachings of which are incorporated herein by reference, this target minimized the amount of test equipment required to project a temperature differential across the surface of a test target. Instead of providing an actual temperature differential across the target, this device provided a target with differential emissivity. Hence, a temperature differential on the target is simulated for the device under test. The emissivity target makes it possible to design a compact, light-weight test unit not achievable with traditional blackbody sources.
The current target employs a glass substrate with a chrome coating. Selective etching of the chrome coating is effective to expose the glass substrate and thereby achieve a tuning of the differential emissivity across the face of the target.
Typical test patterns contain four-bar targets, reticles, and various small apertures. The four-bar targets are used for resolution tests; the reticles are used for aiming and alignment purposes; and, the small apertures are used for testing boresight, scale factors and crosstalks. In many cases, microfeatures that are not discernible by the FLIR are created inside of each bar of the four-bar target to provide a means of varying the average emissivity of the bar. The microfeatures are created by etching an array of small holes through the chrome layer.
For the demands of many current applications, i.e., missile seekers for tracking targets, complete and optimal testing of the device may require a point source at the target which radiates a modulated signal in the short wavelength infrared (0.9 to 1.3 microns) and the long wavelength infrared (8-12 micron) range.
In the conventional design, missile signals of short wavelengths are generated by placing a modulated infrared source behind the small aperture. However, a significant limitation of the conventional target design is its inability to provide a selective modulation for long-wavelength infrared sources. That is, since glass is opaque to long wavelength infrared radiation, to test at these frequencies, small apertures would need to be drilled through the glass substrate. However, since it is difficult to drill such precise holes in the glass substrate, undesirable methods such as electronic shuttering must be employed to simulate long wavelength modulation.
In the conventional design, the modulation of the long wavelength infrared energy can be achieved either by using a mechanical shutter in front of the target or by electronic shutter simulation built into the launcher interface circuit. Mechanical shutter approach can only be used where the target design is very simple and unwanted shuttering of adjacent target signals can be avoided. Although the electronic simulation of the shutter allows more complex target patterns, it will cause all target signals to be shuttered simultaneously. Under this condition, the missile tracker can lock on the designated target only if the target pattern has been carefully laid out such that the surrounding area of each aperture is clear and the missile track box encloses no other target. Such restriction imposed on target layout renders each target design unique to one weapon system. A universal target design essential for future test programs cannot be achieved using the electronic shutter simulation approach. The previous target design can only designate one target and thus cannot meet multiple-tracking test requirements.
Another disadvantage of the conventional target design is due to the difficulty in achieving high irradiance levels and uniformity due to the thickness of the glass substrate which precludes the illuminating source from being placed right behind the aperture. Since the aperture diameter is typically only one-tenth of the substrate thickness, the angular spread of the emitting energy from the aperture is severely limited. The limited angular spread, in turn, can cause irradiance uniformity problems unless the target alignment is tightly controlled using precision assembly fixtures at various stages of assembly build-up. To improve the angular spread, relay optics have to be used. This increases the optical complexity and cost of the target.
A further disadvantage of the conventional target design is the need for a metal plate behind the glass substrate to improve temperature uniformity due to inadequate thermal conductivity of glass. However, the metal plate adds to the fabrication, assembly, alignment and bonding costs of the device.